A cubic boron nitride (hereinafter, also referred to as “cBN”) sintered body employed in a cBN sintered body tool is evaluated as a material that can achieve a long service life at high efficiency by virtue of its chemical stability, low affinity with iron, and high hardness, as compared with conventional cemented carbide tools. Such cBN sintered body tools, when applied to cutting tools, are advantageous in that they have flexibility further superior than that of the grinding tool, low load on the environment, and the like. Thus, cBN sintered body tools have replaced conventional tools in the processing of iron type materials that are difficult to be worked with.
The cBN sintered body is mainly classified into two types of compositions, i.e. high cBN content sintered body and low cBN content sintered body. The former has a high content ratio of cBN particles that directly bind with each other, and the remainder is bound by a binder with Co and/or Al as the main component. The latter exhibits a low ratio of contact between cBN particles due to the low content ratio of cBN particles, and is bound via ceramic having low affinity with iron such as Ti nitrides (TiN) and carbides (TiC). These two types of cBN sintered bodies correspond to different workpieces that are to be subject to cutting work due to the difference in the cBN content ratio. Suitable workpieces for each type of cBN sintered body will be described hereinafter.
During the cutting work of iron-based sintered components governed by mechanical wear and damage caused by the contact with hard grains and during the cutting work of gray cast iron governed by damage caused by thermal shock at the time of intermittent machining at high speed, shear heat caused by swarf does not readily occur since the swarf is easily divided into small pieces. In the cutting work of such material, machining is suitably performed by means of the former high cBN content sintered body. In other words, in the cutting work of gray cast iron or the like, a high cBN content sintered body provides significant stability and long service life by virtue of the superior machine property (high hardness, high strength, high toughness) and high thermal conductivity of cBN.
However, in the case where a high cBN content sintered body is applied to the machining of hardened steel, shear heat will be generated due to the high hardness and continuous swarf. The cutting edge of the high cBN content sintered body is exposed to high temperature, and wear is promptly developed by the reaction between cBN and iron. Thus, a sufficient tool service life cannot be obtained.
Therefore, in the machining of hardened steel, the usage of a low cBN content sintered body is preferable. In other words, a low cBN content sintered body exhibits superior wear resistance particularly at high temperature since it contains a large amount of binder based on TiN or TiC ceramics that has low affinity with iron under high temperature. A tool service life ten to several ten times that of a conventional tool can be achieved. A low cBN content sintered body having such property opened up the cutting market for hardened steel.
Recently in the field of vehicle industry, some of the automobile manufacturers have come to use high-strength cast iron members that are extremely thin, directed to improving the performance and reducing the weight of automobiles. For example, cylinder blocks made of flake graphite cast iron have been modified to cylinder blocks made of vermicular cast iron, and the material of the differential case that is a component of the vehicle has been changed from FCD450 to FCD700. The last three figures of FCD indicate the tensile strength, implying higher strength as the numeric value becomes higher. In accordance with such change of material, the need arises for a tool that can work on high-strength cast iron material at high efficiency and high accuracy.
A material of high strength such as ductile cast iron could be machined only at the rate of 200 m/min. at most using a conventional cemented carbide tool or ceramic tool. Further, the cutting rate was 300 to 400 m/min. at best even if a conventional cBN sintered body tool was used, and the service life of the tool was not of a level that is satisfactory.
For example, Japanese Patent Laying-Open No. 08-120391 (PTL 1) discloses the composition of a cBN sintered body that can cut ductile cast iron with a long service life. Specifically, a long service life of the cBN sintered body is achieved according to PTL 1 by employing carbonitride of any of Hf, TiHf, group IVa element, group Va element, and group VIa element of the periodic table for the main component constituting the binder phase of the cBN sintered body. However, further improvement in wear resistance is required for the cBN sintered body of PTL 1 to satisfy the recent requirement of high speed and long service life.
Further, Japanese Patent Laying-Open No. 2008-222485 (PTL 2) and WO2007/057995 (PTL 3) disclose a covered composite sintered body having a high cBN content sintered body coated with ceramics. However, the wear resistance of the cBN sintered body that is the base material is not sufficient by any of the covered composite sintered bodies. Further improvement in wear resistance is required.
Moreover, Japanese Patent Laying-Open No. 2000-044347 (PTL 4) and Japanese Patent Laying-Open No. 2000-044350 (PTL 5) disclose a cBN sintered body obtained by covering cBN particles with a metal nitride layer such as TiN and AlN, and sintering the same with a material constituting a binder phase.